In the past granting period, we have identified genes and pathways that determine the life span ofyeast mother cells, including SIR2 and SSD1. Importantly, we showed that Sir2p is an NAD-dependent deacetylase, and, as such, mediates the extension in life span due to calorie restriction (CR) in yeast. The C. elegans SIR2 ortholog was also shown to determine life span in that organism. This conservation prompted us to begin a study of the effects of the mammalian SIR2 ortholog, Sirtl, in mice. In particular, we showed that Sirtl is required for at least one phenotype of mammalian CR- the increase in activity induced by this dietary regimen. In the next period, we propose to study the role of Sirtl in life extension induced by CR using several genetically altered mice strains. These include Sirtl knockout (KO) mice, Sirtl transgenic mice, and Sirtl tissue-specific KO mice. We will also examine the effect of CR on Sirtl functional activity and other mammalian physiological parameters. Finally, we will explore the roles of two othermammalian SIR2 homologs, SirtS and Sirt4, which are both mitochondrial proteins. In sum, these studies will shed light on the functions of these Sir2 proteins in mammalian physiology and, in particular, on their role in CR. The roles of specific tissues in CR will also be addressed. The importance of these studies for human health may be large. Understanding the molecular underpinnings of CR and its relationship to mammalian Sirt genes will flesh out the workings of thissalutary diet, the benefits of which have been known for 70 years. Importantly, CR is known to forestall orprevent many diseases of aging in rodent models. Therefore, drugs that are developed against targets we identify may open new strategies to treat diseases of aging.